1. Field of the Invention
This invention relates to a cylindrical filter for precision filtration, prepared by winding up microfine fibers according to a melt-blow process in a cylindrical form, and a process for producing the same.
2. Description of the Related Art
Various kinds of filters have been known, which are obtained by molding synthetic fibers into a cylindrical form. Japanese patent publication No. Sho 56-43139 discloses a process of winding up a carded web of conjugate fibers onto a core under heating. However, according to such a process, it is difficult to subject fine fibers of 1d/f or less to a carding process stably. Hence, a filter collecting fine particles of 10 .mu.m or less could not have been obtained. Further, in the case of conventional synthetic fibers, an oiling agent is coated on the fibers in order to prevent charge and abrasion during the steps of spinning, stretching, carding, etc. This oiling agent causes such problems that it elutes from the fibers into the filtrated during filtration, resulting in bubbling and contaminating the filtrate such as foods, etc.
On the other hand, as cylindrical filters for precision filtration, filters using microfine fibers according to a melt-blow process have been broadly used as a filter for cleaning solutions for materials of electronic equipments or as an air filter for dedusting or as a prefilter for water, etc. used for pharmaceutical products, etc.
Japanese patent application laid-open No. Sho 60-216818 discloses a process of winding up fibers obtained according to a melt-blow process on a core after the fibers have been cooled down to a temperature at which they are not adhered to one another, and also discloses a process of gradually varying the fiber diameter in the thickness direction of the filter by controlling the spinning conditions. In the case of such a filter, the fibers are adhered to one another by entanglements of the fibers almost without bonding of the fibers with one another. Hence, the hardness of the filter is so low that a sufficient pressure-resistance cannot be obtained. In order to increase the hardness, a process of winding up the web while heating it may be considered, but the web is changed into a film due to melting of the fibers, so that clogging of the filtering layer occurs or the size of voids in the filter becomes non-uniform, resulting in a product having an inferior filtration life and accuracy.
Japanese patent application laid-open No. Hei 1-297113 discloses a process of winding up several kinds of non-woven fabrics according to a melt-blow process, each having different fiber diameters and bulk densities, successively each several times so that the inner layer of the filter may be dense and the outer layer thereof may be rough. However, according to the process, it is necessary to prepare several kinds of non-woven fabrics in advance, and not only the production steps are complicated and not efficient, but also the resulting filter is not adhered between the respective fibers of the non-woven fabric and between the respective layers thereof. Hence, solution leakage from the end part of the filter due to peeling-off of the layers during use of the filter are liable to occur and the pressure resistance is insufficient.